A method of the generic type is described in EP 0 654 673 B1, for example. In known measurement methods, a stable, settled state of the measurement current or of a variable derived therefrom as measurement voltage should to be achieved for the evaluation of the measurement. Only once the settled state has been reached are the measurement values for determining the insulation impedance system insulation an additional potential risk and therefore should be avoided. In EP 0 654 673 B1, mean-value generation in the form of an arithmetic mean is used in the detection and evaluation of the measurement voltage.